导航栏加阴影实现

if ([self.navigationBar respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)]){
        [[UINavigationBar appearance] setBackgroundImage:[UIImage p_w_picpathNamed:@"head"] forBarMetrics:UIBarMetricsDefault];
        self.navigationBar.layer.masksToBounds = NO;
        //设置阴影的高度
        self.navigationBar.layer.shadowOffset = CGSizeMake(0, 3);
        //设置透明度
        self.navigationBar.layer.shadowOpacity = 0.6;
        self.navigationBar.layer.shadowPath = [UIBezierPath bezierPathWithRect:self.navigationBar.bounds].CGPath;
    }